The present invention relates to measuring devices for checking an electrical circuit breaker, in particular for checking electric circuit breakers in electrical power generation and transmission plants following commissioning or service inspection of the circuit breaker.
Circuit breakers, which are also referred to as high-voltage switches, are special switches designed for high currents. In energy engineering plants such as, for example, power generation plants such as power stations or power transmission plants, circuit breakers switch not only operating currents, but also overload currents or short-circuit currents in the case of faults. Circuit breakers can be used both to switch on these currents and to switch off the currents. A circuit breaker usually comprises a switching contact and a control drive that mechanically drives the switching contact. The control drive can comprises, for example, spring drives or compressed-air drives, which store mechanical energy that is used for rapid switching of the switching contact. The spring drives or compressed-air drives can be biased or “charged”, for example, by means of electrical drives. The release of the stored mechanical energy for switching the switching contact can be effected by means of so-called trip coils, which are operated electrically. In the electrical plants there is usually a station battery, which provides a direct-current voltage (DC voltage) of between 48 and 200 V, in order to supply the trip coils with energy. Only a floating relay contact is then required for opening or closing. During operation, the commands for opening or closing usually come from protective relays or the instrumentation and control system.
During the checking of an electrical circuit breaker, a so-called micro-ohm test is usually performed, in which, with the switching contact closed, the resistance occurring at the circuit breaker is measured by means of infeed of a predefined current. The predefined current can be of several hundred amperes, for example 200 A. Furthermore, during the checking of the electrical circuit breaker, a so-called timing test can be performed, which registers a switching duration that is required by the switch in order to execute the switching function command after a control command has been applied. Furthermore, a so-called loss-factor test or tan delta test can be performed if the circuit breaker is located in an earth housing, a so-called tank. The loss-factor test is also referred to as a “dead tank test”.
In order to perform the timing test, control commands must be issued, in order to open and close the switch. For this, the trip coils require a certain amount of energy. According to the prior art, this energy comes either from the station battery or from a further device that provides the required DC voltage during testing. Tests are also performed in which the DC voltage is reduced below the nominal value, in order to check whether the switch also works faultlessly with reduced voltage. This test is also referred to as an undervoltage test. This required energy source is referred to in the following as an energy supply unit. The energy supply unit can also be used to recharge the energy storage device in the control drive of the switch. This is effected through application of an alternating current voltage (AC voltage) or DC voltage, usually in the range of from 48 V DC to 230 V AC nominal.
Usually, in order to perform the tests described above, various devices are used, which are connected in succession to the circuit breaker for the purpose of performing the corresponding tests, it also being possible for the energy supply unit to be used at the same time in the case of some tests.